Free-Space High-Speed Laser Communication Link Across the Chesapeake Bay
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Space Systems Development Department
2Research Support Instruments, Inc.
5Optical Sciences Division
Introduction: The development of free-space optical laser communications (FSO lasercomm) is key to DOD's effort to transform the country's National Security Space Infrastructure. FSO lasercomm offers great advantages over conventional RF communications technology. Advantages include higher data rates, low probability of intercept, lower power requirements, smaller packaging, and lower frequency allocation requirements. However, there are also difficulties in implementation of a practical FSO lasercomm system for Naval and space-based platforms: attenuation in the atmosphere, scattering of signal due to atmospheric turbulence, high-precision tracking requirements between moving platforms, etc. The Space Systems Development Department (SSDD) and the Optical Sciences Division have joined forces to investigate and solve many of these difficulties. This joint effort involves the Optical Sciences Division's development of compact and high-efficiency laser amplifiers and high-speed modulators suitable for space-based or Naval platforms1 and the SSDD's development of an FSO lasercomm test bed at NRL Chesapeake Bay Detachment (CBD) to test these components in a realistic environment.2 A maritime environment provides an excellent location to test the worst-case limits of a Naval FSO lasercomm system in a wide range of atmospheric conditions.
FSO Lasercomm Test Bed at CBD: The test bed uses an eye-safe 1550-nm laser operating at 2.5 W, and it consists of a round-trip FSO lasercomm link across the Chesapeake Bay between CBD and NRL-Tilghman Island (Fig. 7). The transmitter and receiver of the lasercomm system are both located at CBD, and an array of corner cube retro-reflectors is located on Tilghman Island. Because only approximately 0.4% of the beam is intercepted by the retro-reflectors, this link is equivalent to a one-way link distance of approximately 72 km.
Background courtesy of MAPQUEST
The received beam is collected with a 16-in. telescope and focused on either a high-speed detector for communication link-quality assessments or a position sensitive detector (PSD) for studies of atmosphere-induced turbulence effects.
High-speed Communication Link: Initial experiments at the CBD-Tilghman Island test bed have successfully demonstrated a communication link operating at up to 500 Mbps. Communication link-quality is measured using a high-speed transmitter and receiver, with modulation rates from 100 to 500 Mbps. The transmitter is modulated with a pseudorandom-bit-sequence (PRBS) from the pattern generator of a bit-error-rate (BER) tester. The received signal is processed in the receiver portion of the tester where a BER is output. The BER of the received signal is measured at 5-s intervals over 2 min for 100, 200, 300, 400, and 500 Mbps. No significant change in average BER is observed between these data rates. Figure 8 is a histogram of the BERs observed at these five data rates. The data show that the BER is below 10-5 89.7% of the time and below 10-4 97.4% of the time.
Atmospheric Turbulence: Turbulent cells in the atmosphere induce significant variation in both the pointing (angle-of-arrival) and intensity (scintillation) of laser beams used in terrestrial FSO lasercomm links. These angle-of-arrival variations and scintillations are the same effects that cause "image dancing" of objects observed over hot surfaces and the flickering of stars. Figure 9 is an example of data acquired with the PSD. This figure shows the angle-of-arrival (top) and scintillation (bottom) of the received beam over a 2-min interval. The angle-of-arrival plot is a histogram of the received spot centroid position on the PSD and the corresponding angle-of-arrival into the receiver telescope. Since high-speed data links typically require small detectors (~10s of microns), these angle-of-arrival variations can cause the received beam to miss the detector and introduce errors in the link. The scintillation plot shows the large variations in received intensity and a particularly deep'"fade" (~20 dB) at 25 s. Intensity scintillations can introduce errors in a link as the results of received intensity levels falling below the detection threshold of the receiver. These turbulence effects are the greatest difficulty associated with a terrestrial FSO lasercomm system, and the mitigation of these effects is the main subject of FSO lasercomm research at NRL and elsewhere.
Future Work: Future work will concentrate on improving the quality, reliability, and speed of the link. The experiments described above do not use active tracking or atmospheric mitigation techniques. A fast steering mirror is being added to the system to reduce angle-of-arrival fluctuations and improve the overall BER and reliability of the link. Higher speed transmitters, receivers, and drive electronics will be used to allow increased data rate transmission, with the ultimate goal of 40 Gbps. Error control coding, adaptive thresholding,3 and other processing techniques will also be implemented to further reduce the BER of the link.
Summary: The establishment of this test bed allows multiple experiments to be performed. These experiments include testing of FSO lasercomm components, studies of atmospheric turbulence and transmission effects on an FSO lasercomm system, atmospheric turbulence mitigation techniques, and maximum achievable data rates as a function of atmospheric conditions. The successful demonstration of this link shows that high-speed FSO lasercomm links in a maritime environment are possible between locations separated by large distances. The link demonstrated here is the longest, highest speed FSO lasercomm link near ground level ever demonstrated.
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